Techniques for the examination of tubular anatomical structures, such as the trachea, arteries, colon, digestive tract, etc., are important procedures for detecting abnormalities. Such abnormalities can include polyps, the latter of which are indicators of developing cancer. For example, optical colonoscopies are an important procedure to detect polyps that are early stage indicators of cancer.
Colorectal colon cancer is the third most common form of cancer and the second leading cause of death among cancers in the western world. Since colorectal cancer is largely preventable, the colonoscopy screening test is recommended for all people age 50 and over. Although optical colonoscopy detects more than 90% of colorectal cancers, it is invasive, uncomfortable, inconvenient, and sometimes can not reach the colon caecum, resulting in an incomplete exam.
As a result, researchers have been investigation virtual colonoscopy (VC) as a computer-based alternative to optical colonoscopy. VC is generally not intended to replace optical colonoscopy, but rather to complement it by providing additional supportive information such as visualizing in both directions, passing high grade stenoses, and planning for surgery. In addition, it is the only alternative offered to those patients that are not comfortable with optical colonoscopy or are severely ill.
The common visualization technique that tries to simulate the real colonoscopy is the virtual fly-through navigation, where a virtual camera with a specific field of view moves along a special planned path inside the colon to render its internal views. The direction of navigation either starts from the colon rectum side (antegrade), or from its caecum side (retrograde). In general, fly-through based-methods suffer from the following limitations: (1) the camera's field of view is limited, and hence results in lower surface visibility coverage; and (2) the navigation must be done in both antegrade and retrograde directions to maximize visualized surface areas, and hence it is very time consuming. Several other visualization techniques have been proposed to overcome those shortcomings, which can be categorized as colon flattening and panoramic methods. The main idea behind colon flattening methods is to initially transform the colon into a cylinder-like shape to reduce its overall tortuosity, and then to map the straightened colon onto a single image, which can be inspected from a single view point. Because flattening methods are based on geometric mapping, geometric distortion is inevitable. As a consequence, important diagnostic features can be altered or even eliminated.
There are a number of panoramic methods. The methods divide the colon into a plurality of views. One of the main drawbacks of this method is that a polyp may split among several of the rendered views. In addition, the layout of several views, such as an unfolded cube is hard to follow, since a medical professional must observe views simultaneously that are sometimes oriented in different directions (horizontal and vertical). Other panoramic methods induce distortion and all have the potential to hide polyps behind haustral folds that can be easily overlooked under the following scenarios: (1) A polyp is located along the camera optical axis, and hence its depth perception is lost by a physician; or (2) A polyp is completely blocked by a haustral fold.